Magnetic resonance imaging is an advanced human body non-invasive imaging technique and widely applied to diagnosis of diseases of all parts of the human body. A magnetic resonance imaging system comprises a magnetic resonance imaging radio-frequency coil, a pre-amplifier for the magnetic resonance imaging radio-frequency coil, and a magnet, a gradient and radio-frequency power amplifier, a spectrometer, a computer and other components. The structure of a magnetic resonance imaging radio-frequency coil in the prior art is an LC resonance loop. The pre-amplifier for the magnetic resonance imaging radio-frequency coil is an important constituent part of the magnetic resonance imaging system, and the performance of the pre-amplifier directly determines the magnetic resonance imaging quality. A magnetic resonance signal is detected by the magnetic resonance imaging radio-frequency coil and input into the spectrometer and the computer of the magnetic resonance imaging system through a system load Rload to be operated after being amplified by the pre-amplifier for the magnetic resonance imaging radio-frequency coil, so that a required image is obtained.
As is shown in FIG. 1 and FIG. 2, a traditional pre-amplifier for a magnetic resonance imaging radio-frequency coil is generally structurally composed of an input matching circuit 1, an amplification circuit 2 and an output matching circuit 3 which are connected in sequence, wherein the amplification circuit 2 comprises an equivalent output impedor Zs, and the two ends of the equivalent output impedor Zs are a first output end and a second output end of the amplification circuit 2 correspondingly; the output matching circuit 3 is an LC circuit and comprises an output capacitor Cm and an output inductor Lm, and the first end of the output inductor Lm is connected with the first output end of the amplification circuit 2. The two ends of the output capacitor Cm are connected with the first output end and the second output end of the amplification circuit 2 correspondingly. The first output end and the second output end of the output matching circuit 3, namely the second end of the output inductor Lm and the second end of the output capacitor Cm, are externally connected with the two ends of the system load Rload correspondingly. A magnetic resonance signal is amplified by the pre-amplifier for the magnetic resonance imaging radio-frequency coil and then input into the spectrometer and the computer through the system load Rload, so that the required image is obtained.
In the pre-amplifier for the magnetic resonance imaging radio-frequency coil, the output matching circuit 3 has an effect of making the impedance value of the equivalent output impedor Zs of the amplification circuit 2 matched with the impedance value of the system load Rload, so that the gain of the pre-amplifier for the magnetic resonance imaging radio-frequency coil is increased. The impedance value of the system load Rload is generally 50 ohms; although the output matching circuit 3 can also partially have a frequency selection effect, since the impedance value of the system load Rload is 50 ohms, the Q value of the output matching circuit 3 is extremely low, and good frequency selectivity cannot be achieved easily.
The operation bandwidth of the pre-amplifier for the magnetic resonance imaging radio-frequency coil of the structure is extremely wide, and a great gain can be achieved within the 20-40 M range near a magnetic resonance signal frequency point. FIG. 3 is a gain curve (obtained by connecting both the input end and the output end to a network analyzer for testing) for independent operation of a typical existing 3.0 T pre-amplifier for a magnetic resonance imaging radio-frequency coil, the gain of the typical existing pre-amplifier at the operation frequency point, namely the maximum gain point or the magnetic resonance signal frequency point of 127.7 M is 28 dB, and the gain of the pre-amplifier for the magnetic resonance imaging radio-frequency coil is kept over 20 dB all the time within the range of 107 M-154 M. Meanwhile, for solving the problem of coupling between channels of a multi-channel magnetic resonance imaging radio-frequency coil, the impedance value of an input matching circuit 1 of the pre-amplifier for the magnetic resonance imaging radio-frequency coil is generally extremely low, and thus the impedance of the magnetic resonance imaging radio-frequency coil is not matched with the impedance of the pre-amplifier for the magnetic resonance imaging radio-frequency coil. After a magnetic resonance signal detected by the magnetic resonance imaging radio-frequency coil is input into the pre-amplifier for the magnetic resonance imaging radio-frequency coil through the input matching circuit 1, a gain curve of the magnetic resonance signal after the combined action of the magnetic resonance imaging radio-frequency coil and the pre-amplifier for the magnetic resonance imaging radio-frequency coil is generally shown as FIG. 4, the gain curve is a curve with two peaks, namely a camel curve, the two peaks are called camel peaks, and gains at the two peaks are extremely high. The operation frequency point of the magnetic resonance imaging radio-frequency coil is generally near the center of the two camel peak frequency points, and the gain at the magnetic resonance signal frequency point is extremely low. Meanwhile, the pre-amplifier for the magnetic resonance imaging radio-frequency coil is generally placed in the magnetic resonance imaging radio-frequency coil, feedback is generated inevitably, and thus the operation frequency point of the pre-amplifier for the magnetic resonance imaging radio-frequency coil is extremely prone to oscillation near the two camel peak frequency points. It is an important technical challenge for those skilled in the field to make the pre-amplifier for the magnetic resonance imaging radio-frequency coil generate a gain required for magnetic resonance imaging only near the magnetic resonance signal frequency point and not generate oscillation at other frequency points.